This is an improvement in the process disclosed in U.S. Pat. No. 4,891,065 in which iron impurities in molten magnesium are lowered by using a binary intermetallic phase of a zirconium material and a silicon material as an iron precipitating agent for the purpose of purifying the magnesium. U.S. Pat. No. 4,891,065 (hereinafter referred to as the '065 patent) is included herein by reference in its entirety.
The Zr/Si binary intermetallic phase is typically formed by contacting a zirconium material and silicon material within a magnesium melt. Whereas the Zr/Si intermetallic phase is effective in reducing the Fe in Mg by precipitation thereof as a ternary intermetallic compound, Fe/Zr/Si, we have found that the presence of dissolved hydrogen in the magnesium hinders or reduces the efficient formation of the Zr/Si intermetallic phase due to the formation of Zr hydride, ZrH.sub.2. Furthermore, the ZrH.sub.2 is formed as a very fine particulate which settles very slowly. The binary intermetallic of Zr/Si and the ternary intermetallic of Fe/Si/Zr settle rapidly due to their high density and favorable morphology. The settling rate of the ZrH.sub.2 is several times slower than that of the binary and ternary intermetallic compounds; the slow settling rate of the ZrH.sub.2 is detrimental to the efficiency of large scale production of the desired high purity, low iron Mg product.
The dissolved hydrogen in the molten Mg can occur as the result of, e.g., electrolytic decomposition of moisture which can enter the electrolytic production of magnesium, from atmospheric humidity which can come into contact with the Mg, (esp. hot or molten Mg), wet melt fluxes, hydrogen-containing species in the molten cell bath, or contact of molten Mg with hydrocarbons. Moisture can react with hot Mg to form MgO and hydrogen. The solubility of hydrogen in the molten Mg increases as the temperature of the Mg is increased. The hotter the molten Mg in the range normally used for producing, holding, or casting the Mg (between about its 650.degree. C. m.p. to about its 1107.degree. C. b.p.) the more hydrogen can dissolve in it, albeit, in low parts per million concentration. The small solubility of hydrogen in molten Mg, expressed in units of weight concentration, is due to the low atomic weight of hydrogen. The solubility of hydrogen in molten Mg, expressed on an atomic ratio basis (atoms of H per atom of Mg), is on the order of about 0.0015 at 775.degree. C., an amount which is not negligible. Cooling of hot Mg to a lower temperature exudes some of the hydrogen that may be in the Mg as an impurity. Even at 650.degree. C., the freezing point of the metal, the Mg can still contain substantial quantities of dissolved hydrogen. The atomic ratio of hydrogen to Mg at 650.degree. C. for a fully saturated metal is about 0.009. However, if a Zr material and a Si material have been added to the Mg, the contaminating hydrogen tends to react with the Zr to form ZrH.sub.2 and interfers with the desired production of Fe/Zr/Si and Zr/Si intermetallics, thus counter-acting, to some degree, the purpose of adding the Zr material and the Si material in the first place.
In order to better assure the absence of hydrogen in the molten Mg, especially in large production vessels where complete exclusion of hydrogen or hydrogen sources is not economically feasible, the present improvement in the process of U.S. Pat. No. 4,891,065 has been developed.
It is an object of this invention to provide an improvement in the process of purifying Mg, especially for use as an essentially pure starting material or reagent for use, e.g., in making U, Ti, Zr, or Mg compounds or Mg alloys with other metals.